Why do oil drain plugs fall out?

I’m not a professional mechanic, but I am a mechanical engineer and do my own car repairs. So it’s mystifying to me when I read reports here that “the drain plug fell out,” as just happened to an unfortunate Rav4 owner.

I can understand a drain plug, say, leaking due to crossed or stripped threads. But falling out? That means someone did screw the plug in far enough that it could hold oil for a while, but didn’t tighten it beyond finger tight. How does that happen? Whenever I install a drain plug I have a wrench in my other hand, and tighten it snug. it doesn’t take much torque to keep the darn plug from falling out, since there’s roughly zero force on the thing.

So how does this happen? Are oil change monkeys so badly trained, distracted, or whatever, that they put a drain plug in by hand and forget to put a wrench on it? Is it that simple?

Maybe some of you pros out there have insight as to how this happens with apparent frequency. Please discuss.

Because nobody respects mechanics. Not customers, not managers, not even other mechanics. I can’t tell you how often someone will walk right into a shop and start talking to a mechanic working on a car. The sheer stupidity of that is lost on someone, yet they would never dream of going to a restaurant, walking right into the kitchen and telling the cook what he wants for dinner.

“Hey Joe, this lady has a question about the brake job you did yesterday.” “I’m busy doing this oil change. She can wait.” “No, she’s a good customer and she’s in a hurry, come talk to her.”

“Hey Joe, give me a hand here, this transmission’s falling off the jack, I’m gonna lose it!!” “Yeah, I’ll be there as soon as I finish this oil change.” “Now dammit, I need help!”

I don’t eat at McDonalds regularly, but I’ve noticed that the order from the drive-thru window is occasionally wrong. A kid starting out working part-time at McDonalds can make more per hour than a kid starting out at QuickyLubeOilMartExpress. So I would imagine that the success rate at McDonalds would be higher than the success rate at LubeBay, based solely on the workers it can draw based on pay. So yes, it can be that simple, as you imply above. You can only train a monkey to do so much. Oil changers are entry-level. We need to weed out the idiots to see who might actually be able to fix a car one day. And the lube rack is where that happens. I don’t like it, but I can’t change an industry.

You want to make sure that your car is serviced by a thorough, experienced, qualified professional? Then pay for it. A $35 oil change gets you a monkey with a crescent wrench. Double that price, you might get some quality.

But that’s not to say that what happened to the RAV4 driver is because of those things. The drain plug simply wasn’t tightened properly. That’s going to happen from time to time. I don’t car who you are or what you do, at some point in your career, in the course of doing your job, you’re going to make a mistake. I make a mistake, a wheel comes off a car. My wife makes a mistake at her job, someone’s payroll isn’t processed on payday. A doctor makes a mistake, my brother has a forgotten drainage tube festering in his abdomen. An engineer makes a mistake, an airplane crashes. It’s as simple as that.

An average mechanic works on 25 cars a week. Suppose he has a 99% perfection rate (who in any job has that?) That leaves one car per month with which something goes wrong. Maybe the RAV4 was that one.

Various reasons; threads damage due to repeated overtightening in the past or present, wrong plug thread if it’s been replaced in the past, or as asemaster correctly mentions; distractions.

When OK had a state vehicle safety inspection program there were 2 reasons why I absolutey loathed doing those inspections.
A. The mechanic got paid 1 dollar for doing the inspection; 2 dollars after the fee schedule was revised and before the program was eventually terminated. The pay did not match the time spent.
B. Distractions from the job at hand.

I can’t even remember how many times I was yanked off of a job and had to perform an inspection which state law required a rack, must be done very promptly, and by the book took an hour of time.

Do the inspection and then one wanders back trying to remember if a certain bolt was tightened or a certain procedure had been done.
A guy I worked with did a ring/head gasket job on a Subaru and had to veer off for an inspection.
He came up to me later with a wristpin circlip and wanted to know if I was screwing around with him. Nope, not me. This led to concerns about whether the distraction had caused him to omit that circlip…

He asked what I would do and my response was go back into the engine (motor still out of car) rather than take a chance. Sure enough, a wristpin was missing the clip and his lucrative customer pay job went south because of this.
State inspections are only one of many distractions that exist in a shop.

I change my own oil and I have to remind myself continuously to tighten the bolt. Even several days later after changing the oil in my son’s car, I couldn’t specifically remember tightening the bolt, so back under to check it again.

This is kind of the sequence for me after getting everything ready:

  1. Use the ratchet to loosen the oil plug. So rachet is in the unscrew mode.
  2. Unscrew the plug by hand and set it aside while the oil drains.
  3. Take the socket off the rachet and put the filter wrench on it and loosen filter.
  4. Unscrew the filter by hand.
  6. Clean the filter plate and hand screw the filter on.
  7. Change the ratchet direction and tighten the filter-NOW I HAVE TO REMEMBER TO TIGHTEN THE PLUG
  8. Put the socket back on the ratchet and tighten the oil plug.
  9. Pull everything out from under the car and put the oil in.

The whole thing only takes about 10 minutes so its just a lot smoother to go back and tighten the plug afterwards, but you have to physically remember to do it. If I’m daydreaming or something and can’t specifically remember tightening the filter or oil plug, I bite the bullet and go back under and check it. Never missed yet, but if in a shop I can see where a guy could get distracted.

“Screws fall out all the time, the world is an imperfect place.” --Judd Nelson’s character John Bender in The Breakfast Club

Drain plugs fall out because the people doing the oil changes often don’t use a fresh crush washer. That is one reason I used to go to the dealership for my oil changes; I knew they used a fresh aluminum crush washer every time.

Another issue is that if someone discovers your drain plug has been cross-threaded or is already damaged in another way, telling you will lead you to blame the current mechanic, not the last one. Nobody likes getting bad news like that, and it’s easier to try to sweep the problem under the rug than it is to tell you the mechanic got your drain plug out, saw that it was damaged, and he can’t finish your oil change until you pay for replacement of the drain pan, which is going to take at least another day since they will have to order the parts, especially if you’re getting your oil changed on a weekend.

This is why I replace my drain plugs with valves (torquing them to the valve manufacturer’s specification) and do my own oil changes.

Oil plugs fall out because the employees that change oil at the quickie-lube places are not mechanics. They have extremely limited mechanical skills and are in a BIG hurry to get you on your way while some other employee tries to sell you overpriced air filters or wiper blades. They don’t tighten it enough, or too much that it strips or they cross thread it. Eventually, this catches up and the plug falls out.

Oil plugs fall out because the employees that change oil at the quickie-lube places are not mechanics. They have extremely limited mechanical skills and are in a BIG hurry to get you on your way while some other employee tries to sell you overpriced air filters or wiper blades.

You don’t need to be a mechanic to do oil change. But you need to be conscientious. The quick lube places are told to move at a very quick pace when they move quickly…mistakes happen. I probably take twice as long as a quick lube place to change my oil…But at least I know it’s done correctly.

@Mustangman, if this were the case, only people who go to quickie-lube places would have this problem. People who go to independent mechanics and dealerships can have this problem too.

@Whitey You aren’t wrong. but many of the independents use the “entry level” employees to change oil, too. Cheap labor to do a simple task. Frees up the real techs to work on more complicated things. Too bad the failure mode is so expensive.


I don’t even touch the oil filter until the oil’s drained completely and I’ve tightened the drain plug back up


I typically do the same on my car, but only because the only way to get to the filter is below the car on the back of the engine. If I didn’t close the drain valve, the oil would drip on me as I change the oil filter.

However, if I had my druthers, I’d prefer to remove the old filter as the oil drains so it can drain faster and so more of it will drain out.

Fortunately, when I do an oil change, I’m in no big hurry. I can let it drain overnight, so by the time I get to the filter, it doesn’t have much oil in it.

Gravity overcomes friction. :slight_smile:

"Drain plugs fall out because the people doing the oil changes often don’t use a fresh crush washer. "

@Whitey, I have to question that. I’ve often reused washers with no problem. All that matters is that the plug is sufficiently torqued. 20 foot-pounds is 20 foot-pounds whether the washer is new or old.

If you’re using a new crush washer, the point between full contact and reaching your torque spec is far larger than if you reuse an old crush washer. That point where the crush washer gives way is what keeps one from stripping the threads and forming a tight seal.

I’ve often reused copper washers, especially on the fill hole of my manual transmission, but true crush washers, made of soft aluminum, should never be used more than once.

If you consider yourself a consummate professional, and you’re doing the job for pay, you really should do it right. Use a fresh crush washer every time.

I think he was referring to nylon washers.

In that case, they might be reusable. I’ve never used them.

I prefer to replace rubber o-rings when I change fluids, but I don’t always do it. If I think ahead, I order new ones when I’m getting other parts.

Nylon washers can probably be reused if they’re not split, but they’re so dirt-cheap to buy by the package that it isn’t worth reusing them. I always kept (still have) packages of nylon washers in various sizes and just tossed them and replaced them. It adds literally only pennies to the cost of the job.

Drain plugs can fall out due to coefficient of expansion.

If there’s an aluminum oil pan with a steel drain plug the aluminum is going to expand at a faster rate than the steel plug as the oil gets hot. So as the drain hole in the pan expands faster than the steel plug, the plug can start to loosen up from the engine vibrations and eventually fall out.


Tester, if the threads are not damaged and the plug properly torqued, the plug should not fall out. If any tech told me that the plug came out due to differences in the coefficient of expansion I’d probably bop him one right on the nose.